Primers, often also called adhesion promoters, are widely known in the form of commercial products or from the technical literature. An overview of the compounds and classes of compound that can be used in primer formulations is found in J. Bielemann, Lackadditive (1998), section 4.3., pp. 114-129.
Primer compositions are disclosed in a host of patent specifications, but only a few specifications describe primers whose aim is to improve the adhesion of adhesive tapes.
Specification WO 2008/094721 A1, in connection with adhesive tape applications, proposes a primer composition based on a maleic anhydride-modified polyolefin and on an organic diamine, the aim of this composition being to improve adhesion to polyolefin-based materials.
JP 2008-156566 A, for adhesive tape applications, discloses a primer composition based on an acidic acrylate polymer and on a fluorine-containing copolymer.
For improving the adhesion of an adhesive tape to substrates coated with melamine resin, WO 02/100961 A1 proposes a primer composition which comprises a graft copolymer of an acrylate copolymer, grafted with an amino alkyl group containing terminal primary amino groups, and further comprising an acrylate copolymer having carboxyl groups in the molecular chain, and a solvent.
WO 03/052021 A1 describes a primer composition which comprises a polydiorganosiloxane-polyurea copolymer having electron-rich groups and which may have the form of a primer, an adhesive, a pressure-sensitive adhesive, or another coating material. This primer composition as well is specified in connection with adhesive tape applications.
Specifications EP 833 865 B1, EP 833 866 B1, EP 739 383 B1, and U.S. Pat. No. 5,602,202 describe primer compositions that are based on mixtures of styrene/diene block copolymers or styrene/hydrogenated diene block copolymers and selected polyacrylates, and which are intended to improve the adhesion of double-sidedly pressure-sensitive foamed adhesive tapes to both low-energy and higher-energy surfaces.
For service as a primer layer within an adhesive tape, WO 03/035779 A describes a primer composition based on a maleinized thermoplastic elastomer, an unhalogenated polyolefin, and a resin.
While the primer compositions described can be used to improve the adhesion of adhesive tapes to certain substrates, there is no known primer which in many cases improves the adhesion of adhesive tapes to particular substrates to such a significant extent that the adhesive tapes can be removed from the substrate after a bonding time of only a few minutes only at the expense of its own destruction or the destruction of the substrate.
More particularly, there is no known primer with which the effects of adhesion improvement mentioned are achieved with adhesive tapes which comprise a foamed or foamlike elastomer layer and are designed for permanent, strong bonds. Moreover, there is no known primer with which the effects of adhesion improvement mentioned are achieved both with adhesive tapes based on polyacrylates and with adhesive tapes based on blends of polyacrylates and synthetic rubber (styrene block copolymers).
It has also been found that a particular difficulty is that of achieving the adhesion-improving effects mentioned on components composed of ribbed PP/EPDM, as illustrated in FIGS. 1 and 2. Ribbed PP/EPDM is found to be more difficult to bond in practice than smooth PP/EPDM. The width of the ribs is normally about 0.8-1.0 mm, although the surfaces of the ribs are not entirely flat, and so the actual width per rib with which a flat adhesive tape is in contact is much lower. The number of ribs per cm of width is about 5 ribs in each case in standard components. Components of the type described are used frequently in the automotive industry, and there is a requirement to bond these to the ribs by means of adhesive tape applications.
Furthermore, there is no known primer with which the effect of improving the adhesion of foamed or foamlike adhesive tapes based on polyacrylates or based on blends of polyacrylates and synthetic rubber is achieved on unpretreated polypropylene in such a way that the adhesive tapes in many cases can be removed from the polypropylene after a bonding time of only a few minutes only at the expense of its own destruction. This is particularly true of adhesive tapes which are designed for permanent, strong bonds and thus have a high internal strength.
Moreover, there is no known primer with which the described effects of improving the adhesion of foamed or foamlike adhesive tapes based on polyacrylates or based on blends of polyacrylates and synthetic rubber are achieved both on the one hand on olefin-based substrates such as PP/EPDM, for example, and on the other hand on substrates such as galvanized steel, ABS and PVC.
Besides the primers described in patent specifications, there are commercial products, such as the 3M Primer 94® or 4298 UV®, for example, for improving the adhesion of adhesive tapes to substrates which are difficult to bond, more particularly both to apolar substrates such as plastics based on polypropylene/ethylene-propylene-diene monomers (PP/EPDM) and to metals such as galvanized steel. A disadvantage, however, is that these primers on ribbed PP/EPDM, especially in combination with foamed or foamlike adhesive tapes based on blends of polyacrylates and synthetic rubber, achieve the effect of improving adhesion only after a prolonged bonding time, if at all, in such a way that the adhesive tapes can be removed from the substrate only at the expense of their own destruction. Moreover, these primers in combination with the adhesive tapes mentioned, especially with adhesive tapes based on blends of polyacrylates and synthetic rubber, do not achieve any promotion of adhesion on unpretreated polypropylene in such a way that the adhesive tapes, after a bonding time of only a few minutes, can be removed from the polypropylene only at the expense of their own destruction.
A further disadvantage of all known primers is that they do not ensure adequate protection against moisture undermining and against corrosion. In the event of relatively long-term storage periods under hot and humid conditions or under extreme fluctuating conditions, such storage periods frequently being required in the automotive, electronics, and solar industries, as for example incorporating temperatures from 60° C. to 90° C. in tandem with a relative humidity of 80% to 90%, moisture undermining generally takes place. In such cases the moisture migrates either between the substrate and the primer or between the primer and the pressure-sensitive adhesive of the adhesive tape, or between both. The consequence is that the adhesion of the adhesive tape is no longer optimum and it can be unwantedly detached adhesively. Moreover, there may be unwanted corrosion, as for example the formation of zinc oxide under the bond area in the case of a galvanized steel substrate.
It is an object of the invention to provide a primer for improving the adhesion of adhesive tapes, especially of adhesive tapes which comprise a foamed or foamlike elastomer layer and are designed for permanent, strong bonds. The improvement in adhesion is to be so significant that the adhesive tapes can be removed from the substrate after a bonding time of only a few minutes only at the expense of its own destruction or the destruction of the substrate.
More particularly, the effect of improving the adhesion is to be achieved both with adhesive tapes based on polyacrylates and with adhesive tapes based on blends of polyacrylates and synthetic rubber (styrene block copolymers).
In addition, the effect of improving the adhesion is also to be achieved on components made from ribbed PP/EPDM (cf. FIGS. 1 and 2). The width of the ribs is typically about 0.8-1.0 mm, although the surfaces of the ribs are not entirely flat, and so the actual width per rib with which a flat adhesive tape is in contact is much lower. The number of ribs per cm of width is about 5 ribs in each case in standard components.
After a number of weeks of storage under hot and humid conditions or under fluctuating conditions, incorporating temperatures of 60° C. to 90° C. in conjunction with relative humidity of greater than or equal to 80% affecting the adhesive tape adhered to the primer-coated substrate, the adhesive tape is to be detachable predominantly only at the expense of its own destruction, and there are to be no instances of moisture undermining, or at least fewer such instances than is the case with the presently known primers.